Stall cell formation over a post-stall airfoil: effects of active perturbations using plasma actuators

Stall cells have been observed over stalled airfoils by many researchers and there have been extensive discussions in the literature on their formation mechanism. They have been known to be unsteady and sensitive to even the smallest upstream perturbations, and therefore, can be made spatially steady by fixing the upstream perturbation location. In the current experimental work, flow with a Reynolds number of 5·10⁵ over a stalled VR-7 airfoil with an aspect ratio 3 was perturbed using a single nanosecond dielectric barrier discharge (NS-DBD) plasma actuator positioned near the leading edge and covering the span of the airfoil. Surface oil flow visualization and stereo particle image velocimetry were employed to investigate spanwise non-uniformities on the airfoil surface as well as in the flow. The results show a gradual appearance of stall cells by increasing the perturbation Strouhal number to approximately 3 times the natural shedding Strouhal number of St_n = 0.6 (St_e ~ 3St_n). The stall cells become well-defined and eventually saturated with no further changes with increasing perturbation Strouhal number beyond 10St_n. The instability responsible for the stall cells is self-sustained as the stall cells persist after the perturbations are stopped. The results of the current work are discussed and interpreted using recent modal and non-modal linear global instability analyses, even though they are carried out in much lower Reynolds number laminar stalled flows over airfoils.